Hi,
I found out that the markup validator says the HTML document is
invalid if it includes the article element(s) which has the pubdate
attribute. It seems that the pubdate attribute is not supported by the
validator yet, but it can be a bug.
Fortunately, this problem is not critical since HTML 5 is still under
its development.
Thanks,
Makoto Mizukami
mmk9060@gmail.com